In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
As disclosed in related U.S. application Ser. No. 09/757,625, the disclosure of which is incorporated herein by reference, binary iron-cobalt (Fe—Co) alloys containing 33-55 wt. % cobalt (Co) are extremely brittle due to the formation of an ordered superlattice at temperatures below 730° C. The addition of about 2 wt. % vanadium (V) inhibits this transformation to the ordered structure and permits the alloy to be cold-worked after quenching from about 730° C. The addition of V also benefits the alloy in that it increases the resistivity, thereby reducing the eddy current losses.
Fe—Co—V alloys have generally been accepted as the best commercially available material for applications requiring high magnetic induction at moderately high fields. Vanadium added to 2 wt. % has been found not to cause a significant drop in saturation magnetization and yet still inhibits the ordering reaction to such an extent that cold working is possible.
Conventional Fe—Co—V alloys employing less than 2% by weight vanadium, however, have undesirable inherent properties. For example, when the magnetic material undergoes a large magnetic loss the energy efficiency of the magnetic material deteriorates significantly. In addition, conventional Fe—Co—V alloys exhibit certain unsuitable magnetic properties when subjected to rapid current fluctuations. Further, as the percentage of vanadium exceeds 2 wt. %, the DC magnetic properties of the material deteriorate.
The composition of conventional Fe—Co—V soft magnetic alloys exhibits a balance between favorable magnetic properties, strength, and resistivity as compared to magnetic pure iron or magnetic silicon steel. These types of alloys are commonly employed in devices where magnetic materials having high saturation magnetic flux density are required. Fe—Co—V alloys have been used in a variety of applications where a high saturation magnetization is required, i.e., as a lamination material for electrical generators used in aircraft and pole tips for high field magnets. Such devices commonly include soft magnetic material having a chemical composition of about 48-52% by weight Co, less than about 2% by weight vanadium, incidental impurities, and the remainder Fe.
U.S. Pat. No. 4,933,026 to Rawlings et al. discloses soft magnetic cobalt-iron alloys containing V and Nb. The alloys include, in wt. %, 34-51% Co, 0.1-2% Nb, 1.9% V, 0.2-0.3% Ta, or 0.2% Ta+2.1% V. Rawlings et al. also mention previously known magnetic alloys containing 45-55% Fe, 45-55% Co and 1.5-2.5% V. The objective of the alloy of Rawlings et al. is to obtain high saturization magnetization combined with high ductility. The ductility and magnetization of the alloy of Rawlings et al. is attributed to the addition of niobium (Nb). Additionally, Rawlings et al. mentions the use of such an alloy in applications such as pole tips and aerospace applications.
U.S. Pat. No. 5,252,940 to Tanaka discloses an Fe—Co—V alloy having a 1:1 ratio of Fe to Co by weight and containing 2.1-5 wt. % V. The Fe—Co—V composition of Tanaka provides high energy efficiency under fluctuating DC conditions by reducing eddy currents.
Fe—Co—V alloys are also disclosed in U.S. Pat. Nos. 3,634,072; 3,891,475; 3,977,919; 4,116,727; 5,024,542; 5,067,993; 5,252,940; 5,443,787; 5,501,747; 5,741,374; 5,817,191; 6,146,474 and 6,225,556 the disclosures of which, as they are related to thermomechanical processing of such alloys, are hereby incorporated by reference.
According to an article by Phillip G. Colegrove entitled “Integrated Power Unit for a More Electric Airplane”, AIAA/AHS/ASEE Aerospace Design Conference, Feb. 16, 1993, Irvine, Calif., an integrated power unit provides electric power for main engine starting and for in-flight emergency power as well as for normal auxiliary power functions. Such units output electric power from a switched-reluctance starter-generator driven by a shaft supported by magnetic bearings. The starter-generator is exposed to harsh conditions and environment in which it must function, e.g., rotational speeds of 50,000 to 70,000 rpm and a continuous operating temperature of approximately 500° C. The machine rotor and stator can be composed of stacks of laminations, each of which is approximately 0.006 to 0.008 inches thick. The rotor stack can be approximately 5 inches in length with a diameter of approximately 4.5 inches and the stator outside diameter can be about 9 inches. HiSat-50, an alloy produced by Telcon Metal Limited of England, has been proposed for the rotor and stator laminations annealed at a temperature providing a desirable combination of strength and magnetic properties. The magnetic bearings are operated through attraction of the shaft toward the magnetic force generator. The bearings exhibit a desirable combination of stiffness and load capability as well as compatibility with requisite operating temperatures and operational frequencies. The operational temperature of the bearings, for example, can be on the order of 650° F.
Iron-cobalt alloys have been proposed for magnetic bearings used in integrated power units and internal starter/generators for main propulsion engines according to an article by Richard T. Fingers et al. entitled “Mechanical Properties of Iron-Cobalt Alloys for Power Applications” published in the 32nd Intersociety Energy Conversion Engineering Conference Proceedings, Vol. 1, p. 563 (1997). Two iron-cobalt alloys investigated include Hipercoh alloy 50HS from Carpenter Technology-Corporation and HiSat-50 from Telcon Metal Limited. After heat treating at 1300 to 1350° F. for 1 to 2 hours, tensile properties were evaluated for specimens prepared from rolled sheet 0.006 inches thick. Both materials are categorized as near 50-50 iron-cobalt alloys having a B2-ordered microstructuire but with small percentages of vanadium to increase ductility, and other additions for grain refinement. The Hiperco™ alloy 50HS is reported to include, in weight percent, 48.75% Co, 1.90% V, 0.30% Nb, 0.05% Mn, 0.05% Si, 0.01% C, balance Fe; whereas HiSat-50 includes 49.5% Co, 0.27% V, 0.45% Ta, 0.04% Mn, 0.08% Si, balance Fe. The alloys annealed at 1300° F. are reported to exhibit the highest strength while those annealed at 1350° F. exhibit the lowest strength. According to the article, in developing motors, generators and magnetic bearings, it will be necessary to take into consideration mechanical behavior, electrical loss and magnetic properties under conditions of actual use. For rotor applications these conditions are temperatures above 1000° F. and exposure to alternating magnetic fields of 2 Tesla at frequencies of 5000 Hz. Furthermore, the clamping of the rotor will result in large compressive axial loads while rotation of the rotor can create tensile hoop stresses of approximately 85 ksi. Because eddy current losses are inversely proportional to resistivity, the greater the resistivity, the lower the eddy current losses and heat generated. Resistivity data documented for 50HS annealed for 1 hour at temperatures of 1300 to 1350° F. indicate a mean room temperature resistivity of about 43 micro-ohm-cm whereas a value of 13.4 micro-ohm-cm is reported for HiSat-50 annealed for 2 hours at temperatures of 1300 to 1350° F. The article concludes that both alloys appear to be good candidates for machine designs requiring relatively high strength and good magnetic and electrical performance.
Conventional Fe—Co—V based soft magnetic alloys are used widely where high saturation magnetization values are important. However, their yield strengths are low at room temperature, and the yield strengths are even lower at high temperatures, making the alloys unsuitable for applications such as magnetic parts for jet engines that impose high temperatures and centrifugal stress on materials. Accordingly, there is a need for low cost alloys having improved strength (both at room temperature and elevated temperatures), improved creep resistance, and increased resistivity that retain good magnetic properties.